Need help- formulating difference equation

there is a square with 4 equal blocks. if a mosquito set to fly starting at block 1, it is equally likely to fly to other blocks 2,3,4. what is the probability after n flies, the mosquito is at the 4th block(which is diagonally opposite). form a difference equation.

Can I use the law of total probability to solve it. Can anybody help solving it. Thanks.